In recent years, there has been a growing tendency for environmental protection, and therefore an attempt has been made to reform a digestion gas or a biogas yielded in methane fermentation or the like of organic wastes into a hydrogen-containing gas which is then utilized to generate electricity in a fuel cell. Conventionally, for example, a digestion gas is pretreated to remove hydrogen sulfide and carbon dioxide therefrom for thereby enriching the concentration of methane in the gas, and the gas with a higher methane concentration is then reformed to produce a hydrogen-containing gas having a hydrogen content of 70 to 80%. The produced hydrogen-containing gas is then supplied as a fuel gas to the anode of a fuel cell while air is supplied as an oxidizing gas to the cathode of the fuel cell, thus generating electricity.
In general, a digestion gas produced in methane fermentation of sludge at a sewage treatment plant is pretreated by a method of wet absorption in which hydrogen sulfide and carbon dioxide are absorbed and separated by a large amount of treatment water from the sewage treatment plant. For a digestion gas produced in methane fermentation of organic matter other than the sludge in the sewage treatment, since a large amount of treatment water does not exist, hydrogen sulfide is removed by a method of dry adsorption with solid sorbents such as iron oxide and carbon dioxide is removed by membrane separation or pressure swing adsorption (PSA).
Further, in the pretreatment of the digestion gas, an attempt has also been made to perform only desulfurization without the separation of carbon dioxide, i.e., without the enrichment of methane.
The method of wet absorption with a large amount of treatment water has an advantage that no chemical is necessary. However, the carbon dioxide absorption capacity of the treatment water is so small that the size of the absorption apparatus has to be very large. Further, the dissolved oxygen and dissolved nitrogen in the treatment water are transferred to the gas after the treatment, and therefore oxygen and nitrogen contained in the gas after the absorption amount to several percents, thus limit the level of the enrichment of methane, and adversely affect the subsequent hydrogen production step and the fuel cell power generation step.
On the other hand, in the PSA or the membrane separation, power for increasing or decreasing the pressure of gas is necessary, and, in addition, the recovery rate of methane is low, disadvantageously leading to a lowered energy efficiency of the system. As for the dry adsorption of hydrogen sulfide, although the apparatus can be simplified, a high adsorption load of hydrogen sulfide causes an increase in running cost.
Further, in the case where the pretreatment does not involve the separation of carbon dioxide, the power necessary for blowing and pressurizing the digestion gas to the subsequent step of reforming is increased. In addition, the concentration of hydrogen in the reformed gas or hydrogen-containing gas is considerably low, and this poses a problem that the energy efficiencies of the reforming step and the fuel cell power generation step are disadvantageously lowered.
Further, the concentration of methane in digestion gas varies from fermentation tank to fermentation tank used for the production of methane. Even in the same fermentation tank, the methane concentration varies with the season or a fluctuation in fermentation conditions or the like, and the difference between the minimum concentration and the maximum concentration may reach 10%. A fluctuation of the methane concentration, i.e., the heating value of the gas supplied to the reforming step not only unstabilizes the operation of the hydrogen production step and thus the operation of the fuel cell power generation step, but also significantly impairs the energy efficiency of the total system. Therefore, it is important to keep the concentration of methane in the feed gas at a constant value as much as possible. In the conventional technology described above, however, it has been difficult or impossible to cope with this requirement.